Atmospheric reconnaissance of the habitable-zone Earth-sized planets orbiting TRAPPIST-1 (1802.02250v1)

Abstract: Seven temperate Earth-sized exoplanets readily amenable for atmospheric studies transit the nearby ultracool dwarf star TRAPPIST-1 (refs 1,2). Their atmospheric regime is unknown and could range from extended primordial hydrogen-dominated to depleted atmospheres (refs 3-6). Hydrogen in particular is a powerful greenhouse gas that may prevent the habitability of inner planets while enabling the habitability of outer ones (refs 6-8). An atmosphere largely dominated by hydrogen, if cloud-free, should yield prominent spectroscopic signatures in the near-infrared detectable during transits. Observations of the innermost planets have ruled out such signatures (ref 9). However, the outermost planets are more likely to have sustained such a Neptune-like atmosphere (refs 10,11). Here, we report observations for the four planets within or near the system's habitable zone, the circumstellar region where liquid water could exist on a planetary surface (refs 12-14). These planets do not exhibit prominent spectroscopic signatures at near-infrared wavelengths either, which rules out cloud-free hydrogen-dominated atmospheres for TRAPPIST-1 d, e and f, with significance of 8, 6 and 4 sigma, respectively. Such an atmosphere is instead not excluded for planet g. As high-altitude clouds and hazes are not expected in hydrogen-dominated atmospheres around planets with such insolation (refs 15,16), these observations further support their terrestrial and potentially habitable nature.

Atmospheric Reconnaissance of the TRAPPIST-1 System

The research paper under review presents a comprehensive atmospheric analysis of the terrestrial exoplanets orbiting the ultracool dwarf star TRAPPIST-1. The central focus is on identifying the atmospheric compositions of these planets, which are located in or near the star's habitable zone (HZ) where liquid water might be possible. This is accomplished through a series of transit observations using the Hubble Space Telescope's (HST) Wide Field Camera 3 (WFC3).

Key Findings

The collaborative paper investigates the atmospheric signatures of four of the seven known Earth-sized planets in the TRAPPIST-1 system: planets d, e, f, and g. These specific planets are situated in or near the HZ. The observations did not detect prominent spectroscopic features in the near-infrared range that would be indicative of clear, hydrogen-dominated atmospheres. For planets d, e, and f, such atmospheres were ruled out with significance levels of 8, 6, and 4 sigma, respectively. Planet g remains less definitively constrained in this analysis.

The implications are that these planets are more likely to have atmospheres composed of heavier molecules, potentially similar to Earth's, or atmospheres dominated by aerosols which mask the signatures of hydrogen. The presence of hydrogen as a major atmospheric component would significantly impact the planets' surface conditions and their potential habitability.

Methodology

The research relies on advanced spectroscopic techniques to derive transmission spectra from transit light curves. By employing multiple transit-per-visit strategies, the researchers optimized observational efficiency, crucial given the limited visibility window of TRAPPIST-1 from Earth.

A notable challenge was the presence of the South Atlantic Anomaly (SAA), impacting data due to a loss of pointing precision during HST orbits affected by the SAA. Refinements in data processing addressed these challenges, yielding consistent results that corroborate with prior studies on TRAPPIST-1 b and c.

Significance and Future Directions

The absence of hydrogen-dominated atmospheres in TRAPPIST-1's HZ planets enhances their candidacy for further exploration of terrestrial and potentially habitable worlds beyond our Solar System. The paper suggests theoretical consistency with atmospheres composed of N$_2$, CO$_2$, or H$_2$O-dominated environments, although aerosol interference remains a possibility.

These findings are pivotal in refining protocols for exoplanet atmospheric characterization and underscore the need for next-generation observatories like the James Webb Space Telescope (JWST). Advances in detection capabilities will allow more detailed aerosol characterization and the probing of atmospheres with greater mean molecular weights.

This paper contributes significantly to exoplanet science by refining our understanding of atmospheres surrounding Earth-sized exoplanets in habitable zones. It also provides context for theoretical models regarding the formation and evolution of atmospheres around low-mass stars. Future exploration, particularly with the JWST, promises to deepen this understanding and expand paradigms related to exoplanetary habitability.